1. Field of the Invention
The present invention relates to an optical transmitter that implements a wavelength tunable laser diode (wavelength tunable LD) and a method of controlling the same.
2. Background Arts
Various prior documents has disclosed techniques for reducing frequency noises inherently contained in laser light output from a semiconductor laser diode (wavelength tunable LD), which is equivalently to reduce a linewidth of the laser light. One of such techniques re-enters a portion of the laser light into a cavity of the wavelength tunable LD by reflecting the laser light out of the wavelength tunable LD. Another technique feeds an output of a wavelength dependence filter that transmits the laser light back to a current for adjusting a phase of the laser light to decrease the linewidth of the laser light. Still another technique re-enters a portion of the laser light into the cavity after rotating the polarization of the laser light.